rockchip: spl-boot-order: do not attempt to access fdt if OF_PLATDATA

gd->fdt_blob is null if using OF_PLATDATA in SPL, which causes a hang
after f0921f50 ("fdt: Sync up to the latest libfdt").
We use the same test that is used in spl_common_init on whether to call
fdtdec_setup to unconditionally avoid linking in the fdt-using code
when not necessary and thus reduce SPL size.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>